The present invention relates generally to controls for gas appliances, and more particularly to a drive circuit for triggering an ignitor of a gas furnace.
Typical gas furnaces manufactured today include some type of electronic ignition system. The most common type incorporate hot surface ignitors which produce high temperatures (e.g., 2,000 degrees Fahrenheit) for burner ignition within the furnace. The ignitor, when activated, ignites gas flow at the main burner of the furnace without the use of a pilot light. These electric ignition systems increase the efficiency of the furnace, thereby increasing the efficiency of the HVAC system to which they are connected. Further, gas ignition is provided without the need for a continuously burning pilot light or flame.
However, with the extreme variations in temperature experienced by the hot surface ignitors, failure is more likely. New designs and material for use in these ignitors have been developed to extend the life of the ignitors and provide increased performance. For example, the Intell-Ignition(trademark) integrated ignition system manufactured and sold by the White-Rodgers Division of Emerson Electric Co. includes a silicon nitride ignitor providing added strength and durability, thereby resulting in longer useful ignitor life. These ignitors also provide optimization of ignition temperature. Other types of hot surface ignitors may be constructed of various materials including aluminum nitride, silicon nitride, silicon carbide, boron carbide, tungsten disilicide, tungsten carbide, and mixtures thereof.
A problem with silicon nitride and similar ignitors is that in order to maintain proper operating temperature, the ignitor switch (e.g., a triac) must be constantly triggered (i.e., turned on and off) to control the current and voltage to the ignitor. Also, AC voltage is preferably provided to the ignitor in order to minimize material migration of ignitor elements, which migration may result from the ignitor brazing material (i.e., sliver) migrating from the positive ignitor electrode and negative ignitor electrode, thereby causing a short across the electrodes and failure of the ignitor. Thus, minimizing material migration extends the ignitor life. However, as a result of the constant triggering of the AC voltage, line conducted interference (i.e., electromagnetic interference (EMI)) results, particularly if the switch is triggered late in the line voltage cycle (i.e., +/xe2x88x9215 volts). This late triggering at too high a voltage causes a high ramp-up rate of the current through the switch to the ignitor with a corresponding current spike and accompanying EMI. This EMI not only can damage the component parts of an electronic ignition (e.g., a microcomputer or the ignitor), but also can result in other perceptible annoyances with surrounding systems, such as causing light flicker and interference with AM radio signals and RF controls. With the ignitor controllers and drive circuits presently available, triggering of ignitors occurs too late in the line voltage cycle (i.e., +/xe2x88x9215 volts), thereby resulting in excessive line conducted EMI. The known ignitor controllers for controlling ignitor ignition include triac drivers. However, these drivers fail to consistently trigger ignitor switches close enough to the zero-crossing of the line voltage to sufficiently reduce EMI. Line filters are also used to reduce EMI, however, such filters are costly.
The present invention provides an ignitor controller and method of providing the same for reducing EMI in gas appliances, and specifically gas furnaces using silicon nitride and similar ignitors. The EMI results from the constant switching of AC power to these types of ignitors that is required to maintain acceptable temperature levels for proper ignition and operation. Specifically, the invention provides a controller including a drive circuit configured to trigger an ignitor switch in order to reduce line-conducted interference. The controller of the present invention provides trigger pulses to the igniter switch (e.g., a triac) controlling an ignitor ignition, such that the triac is triggered after the zero-crossing of the line voltage to the triac, but before the line voltage increases to a point where excessive EMI results.
Succinctly, the present invention provides an ignitor controller having a zero-crossing triac drive circuit for triggering an ignitor of a gas furnace. The inventor has determined that in order to sufficiently reduce EMI levels, a triac controlling a silicon nitride ignitor is preferably triggered after the AC line voltage crosses a zero-voltage point, but prior to reaching +/xe2x88x92five volts.
A zero-crossing triac drive circuit constructed according the principles of the present invention essentially comprises a series switch, a two-transistor zero-voltage detector circuit, and an energy storage device to ensure triggering after the zero-crossing point. As part of an integrated control system for an HVAC unit, the series switch may be provided in series with an optocoupler connected to a microcomputer which can enable or inhibit the triac drive pulse from the series switch as required by the system. The optocoupler provides electrical isolation between the low voltage portion of the HVAC system (i.e., 24 volt thermostat control) and the high voltage portion (i.e., the system components requiring line voltage (e.g., 110 volts) to operate, such as the furnace). The series switch is turned xe2x80x9coffxe2x80x9d by the conduction of one transistor of the zero-voltage detector circuit during the positive phase of the AC line voltage and the other transistor of the zero-voltage detector circuit during the negative phase of the AC line voltage.
The ignitor controller of the present invention not only sufficiently minimizes EMI levels, but also reduces the cost of manufacture and operation, while requiring less physical space to implement and construct.
While the principal advantages and features of the present invention have been explained above, a more complete understanding of the invention may be attained by referring to the description of the preferred embodiments which follow.